MacOS folder action to reduce screenshots size automatically

pngminify.zsh

#!/bin/zsh
# This can be used as a folder action on the macOS's desktop to reduce the size of
# most PNG files automatically.
# 
# http://about.me/lestercheung

POSTERIZE=0
PNGQUANT=0
[[ -x /usr/local/bin/posterize ]] && POSTERIZE=1
[[ -x /usr/local/bin/pngquant  ]] && PNGQUANT=1
if [[ $POSTERIZE -eq 0 ]]; then
    cat <<EOF | /usr/bin/osascript
display alert "You need pngquant installed:\nbrew install pngquant "
EOF
fi
if [[ $PNGQUANT -eq 0 ]]; then
    cat <<EOF | /usr/bin/osascript
display alert "You need posterize installed. Bringing you to the github repo..."
open location "https://github.com/kornelski/mediancut-posterizer"
EOF
fi

TMPFILE=`mktemp /tmp/convert.XXXXXX`

for f in $*; do
    echo $f >> /tmp/log.pngminify
    [[ "$f" != *".png" ]] && continue
    if [[ POSTERIZE -eq 1 && PNGQUANT -eq 1 ]]; then
        echo converting "$f"
        /usr/local/bin/posterize -Q 80 "$f" | /usr/local/bin/pngquant - > "$TMPFILE"
        if [[ $? -eq 0 ]]; then
            if [[ -x /usr/local/bin/trash ]]; then
                /usr/local/bin/trash "$f"
            else
                /bin/rm "$f"
            fi
            /bin/mv "$TMPFILE" "$f"
        fi
    fi
done

[[ -f "$TMPFILE" ]] && /bin/rm "$TMPFILE"